                (a) Field of the Invention        
The present invention relates to a signal transmission line for an optical modulator and, more particularly, to the improvement of the signal transmission line for transmitting a high-frequency modulating signal to the optical such as an electro-absorptive modulator (EA modulator) suitably used in an electro-absorptive-modulated laser (EAML) module.
(b) Description of the Related Art
The EAML module has an integrated optical device including a distributed-feedback (DFB) laser diode and an associated EA modulator, which are integrated on a common semiconductor substrate. The EAML modules now attract increasing attention for use in an optical communication system due to the smaller occupied area and the lower fabrication cost thereof.
FIG. 17 shows a conventional EAML module described in Patent Publication JP-A-9(1997)-252164. The EAML module 10A includes a chip of an integrated optical device 11 encapsulated in an air-tight package 12 having thereon a plurality of external pins 13. A high-frequency modulating signal input through one of the external pins 13 is transferred through a signal transmission line (assembly) 14 to the EA modulator in the integrated optical device 11. The signal transmission line 14 includes a micro-strip line 15 and an aluminum nitride carrier mounting thereon the micro-strip line 15, the aluminum nitride carrier being mounted on a chip carrier 16 mounting thereon the integrated optical device 11 together with the signal transmission line 14.
The micro-strip line 15 is connected to the external pin 13 via a bonding wire 19, and also connected to the EA modulator in the integrated optical device 11 via a bonding wire 20. A termination resistor 21 is connected to the end of the micro-strip line. The optical output of the integrated optical device 11 is coupled to an optical fiber 22 for transferring the modulated optical signal. A photodiode 23 is optically coupled to the rear end of the DFB laser diode in the integrated optical device 11 for measuring the level of the optical output power from the laser diode.
FIG. 18 shows the equivalent circuit diagram of the signal transmission line 14 transferring the high-frequency modulating signal from the signal source to the EA modulator. The output impedance of the signal source 40 is designed at 50 Ω, the micro-strip line 15 has a characteristic impedance of 50 Ω, and the EA modulator 24 is substantially equivalent to a capacitor having a capacitance of 0.7 pF, for example. The termination resistor 21 has a resistance Rt of 50 Ω.
In operation, the combination impedance of the EA modulator 24 and the termination resistor 21 has a resistance of around 50 Ω in a lower frequency range, which matches with the output impedance (50 Ω) of the signal source 40. On the other hand, in a higher frequency range around 10 GHz or above, the combination impedance significantly reduces below 50 Ω due to the reduction in the impedance of the EA modulator 24 having a capacitive impedance. The reduction of the combination impedance causes impedance mismatching of the overall input impedance of the EA modulator 24 including the signal transmission line 14 with the output impedance (50 Ω) of the signal source 40.
The signal reflection is generally evaluated by the term “input return loss” which is defined by a ratio (dB) of the signal power returned from the EAML module 10A toward the signal source 40 to the total signal power supplied from the signal source 40. The input return loss decreases the efficiency of the signal conversion from the input electric signal to the output optical signal in the EA modulator, and thus it is desired to reduce the input return loss at the desired frequency range. It is prescribed in the industrial RF specifications of the optical communication system that the input return loss be −10 dB or lower at the operating frequency thereof. This should be achieved together with a specified (−3 dB) modulation bandwidth of around 10 GHz or above, for example.
In a high-frequency range, the maximum power transfer cannot be achieved unless the overall impedance of the load, i.e., EA modulator 24 including the signal transmission line 14, is a conjugate of the source impedance. Thus, the impedance matching is attempted in the conventional technique; however, it is difficult in fact to achieve the impedance matching in the desired frequency range of the EA modulator, or optical modulator in general.